Photolithography is presently employed in sub-micron resolution integrated circuit (IC) manufacturing, and, to an increasing degree, in advanced, wafer-level IC packaging technologies, and in micro-mechanical systems (MEMS), nano-technology, among other applications. These applications require multiple imaging capabilities ranging from relatively low resolution, for example a few micrometers (μm), with large depth of focus, to relatively high resolution, for example sub-μm, and with high throughput. A commonly used projection optical system for these applications is a unit-magnification projections lens.
A unit-magnification, imaging, catadioptric, single-reflection optical system, consisting of a spherical mirror and a plano-convex lens, is described in a paper by J. Dyson, entitled “Unit magnification optical system without Seidel aberrations,” J. Opt. Soc. Am. 49(7), pp. 713-716 (1959). In this single-reflection optical system, there is an aperture stop at the mirror, and the axial thickness of the plano-convex lens is equal to the radius of curvature of its convex surface. The lens is spaced apart from the mirror such that the centers of curvature of the spherical surfaces of the mirror and the lens are concentric and lie on the optical axis of the object and image planes. The radius of curvature of the mirror and the convex surface of the lens are chosen such that the Petzval sum of the optical system is zero. Such a concentric system is paraxially telescopic or telecentric in the object and image spaces. The object and image fields of this unit-magnification Dyson system are mutually inverted and lie on the rear plane surface of the lens. This system is well corrected for Seidel aberrations, i.e., no third-order monochromatic aberrations, but the lens contributes substantial higher-order aberration for off-axis field points, in addition to chromatic aberrations when used over an extended spectral range. The Dyson system has been used to image one half of the full image plane surface onto the other half. It has been used as projection optical system for photolithography for small field, narrow spectral-band exposure systems.
A modified Dyson system is described by C. G. Wynne in articles “A unit power telescope for projection copying,” Optical Instruments and Techniques, Oriel Press, Newcastle upon Tyne, England (1969), and “Monocentric telescope for microlithography,” Opt. Eng. 26(4) 300-303 (1987). Wynne modified the Dyson system and extended its optical performance by using a doublet lens consisting of a monocentric negative meniscus element cemented to a plano-convex lens element. This unit-magnification Wynne-Dyson optical system provides very high aberration correction over an extended field of view at numerical aperture greater than 0.30, and over quite a wide spectral range. Correction from 546 nm to 405 nm is possible for a system designed to work in the visible spectrum, where a wide range of optical glasses is available.
Like the Dyson system, the plane surface of the doublet lens of the Wynne-Dyson system is imaged, inverted, on itself. In practice, the object is generally placed in one half of the object/image plane with the image appearing on the other half. Wynne described two practical methods of separating and transferring these object and image planes to more convenient positions. The first method is to convert part of the thick glass lens block into two identical folding prisms. This provides good access to both the object and image planes but the cost of this gain is the substantial reduction of available object/image field size. This method of field division was used on Wynne-Dyson type optical systems described in several patents including U.S. Pat. Nos. 4,391,494, 4,171,871, 4,103,989, 6,813,098, 6,879,383, 7,116,496, and 7,148,953. A second method, which provides a larger imaging field area but with considerable loss of light, inserts in the rear glass block a semi-reflecting surface at 45° to the optical axis, forming a beam-splitter. The use of the beam-splitter enables the separation of the object and image planes without sacrificing the field size. The beam-splitter method of separating the object and image surfaces is used in Dyson systems described in several patents, including U.S. Pat. Nos. 4,171,870, 4,302,079, 3,536,380, and 2,231,378.
The unit-magnification optical systems described in the above-referenced patents have working distances (air spaces between an outermost optical element and object or image planes) ranging from a fraction of a millimeter (mm) to a few millimeters. These systems are suited for photolithography applications in the ultraviolet and deep ultraviolet regions of the electromagnetic spectrum. While the projection lens designs described in these above referenced patents are quite suitable for normal photolithography aspects at wavelengths of 404 nanometers (nm), 365 nm and 248 nm and a 0.35 NA exposure system, such lens designs have not provided capabilities at large working distances and diode-laser wavelengths, for example, 808 nm, 980 nm, and 1024 nm.
The design embodiments described in the above-referenced patents are not suitable for exposure systems requiring large rectangular exposure fields with lengths ranging from 100 mm to a few hundred mm, and with working distances of at least 100 mm. Such field dimensions and working distances are required for masked laser-patterning apparatus used in the manufacture of liquid crystal, LED, and OLED display panels or screens.
For theses applications, it is desirable to provide optical designs of large-field unit-magnification projection optical systems capable of imaging, in one exposure, large rectangular object fields with lengths greater than 100 mm, and having working distances greater than 100 mm. This would significantly increase system throughput in masked laser-patterning apparatus.